In recent years, both the size and weight of electronic equipment such as a notebook-sized personal computer are being reduced to increasing extents. Accordingly, IC wiring is also becoming finer.
With respect to the wiring pattern formed on a substrate used in such an electronic equipment, the lead width is now as small as ten-odd microns (.mu.m). In accordance therewith, the metal foil constituting the wiring pattern is becoming thinner. Specifically, while the designated thickness of metal foil for use in the formation of the conventional wiring pattern of about 100 .mu.m lead width has ranged from about 15 to 35 .mu.m in correspondence to the width of the wiring pattern, the thickness of metal foil employed in the formation of ten-odd micron (.mu.m) wiring pattern must be reduced in correspondence thereto.
For example, an aluminum foil or a copper foil is used as the metal foil for constituting the above wiring pattern. It is preferred to employ a copper foil, especially an electrodeposited copper foil, as the metal foil.
The electrodeposited copper foil employed for forming the above wiring pattern is produced by electrodepositing copper on a drum surface. With respect to the thus produced electrodeposited copper foil, the surface at which copper deposition is initiated, namely the surface in contact with the drum, is referred to as "shiny side", and the surface at which copper deposition is completed is referred to as "matte side". The surface condition of the shiny side is substantially the same as that of the drum. That is, the 10-point average surface roughness (Rz) of the drum is about from 1.2 to 2.5 .mu.m, to which the 10-point average surface roughness of the shiny side is nearly equal. On the other hand, with respect to the matte side, its surface roughness is greater than the surface roughness of the shiny side, and the 10-point average surface roughness of the matte side, although varied depending on the deposition condition of copper and the thickness thereof, is generally in the range of about 2.5 to 10 .mu.m. In the conventional electrodeposited copper foil of about 35 .mu.m nominal thickness, it has been rare that the surface roughness of the matte side poses a problem. However, in the electrodeposited copper foil of ten-odd micron (.mu.m) thickness, the surface roughness of the matte side is equivalent to tens of percents of the thickness of the whole electrodeposited copper foil, and the condition of the matte side exerts marked influence on the electrical properties of formed wiring pattern and board per se. It is known that, for example, mechanical polishing, chemical polishing and electrolytic polishing are available as the means for preparing the state of surface of the copper foil. The mechanical polishing is a method of smoothing the surface of the copper foil with the use of, for example, a buff. When use is made of a thin copper foil, the copper foil may be broken by mechanical stress exerted on the copper foil. Thus, the mechanical polishing is suitable for the conditioning of the surface of relatively thick copper foils. On the other hand, no mechanical stress is exerted on the copper foil in the chemical polishing and electrolytic polishing, as different from the mechanical polishing, so that even relatively thin copper foils would not be broken by the chemical polishing and electrolytic polishing. Thus, it has been believed that the chemical polishing and electrolytic polishing are suitable for the preparing (conditioning) of the surface of relatively thin copper foils.
For example, Japanese Patent Application Publication (Unexamined) No. Hei 5-160208 discloses a tape carrier having a lead pattern formed from an electrodeposited copper foil wherein the overall surface of matte side obtained by electrodeposition has been prepared (conditioned). This publication discloses the use, in the formation of a lead pattern of 60 to 80 .mu.m pitch, of an electrodeposited copper foil whose matte side surface of 1-2 .mu.m has been chemically polished. The thickness of the there employed electrodeposited copper foil after the polishing is in the range of 18 to 30 .mu.m. It is disclosed that a highly reliable carrier tape with desired lead strength can be provided by the use of the copper foil whose matte side overall surface has been chemically polished.
However, the preparing of copper foil by chemical polishing as described in the above publication, although protrudent parts of the matte side are leached with relatively high selectivity to thereby effect preparing thereof, also invites leaching of the copper constituting the depressed parts of the matte side. Therefore, in this chemical polishing, the whole copper foil tends to become thin. Accordingly, when the thin electrodeposited copper foil employed in conformity with the recent trend toward fine pitch, for example, the electrodeposited copper foil having a thickness of 35 .mu.m (1 ounce), or 17.5 .mu.m (1/2ounce), or less is chemically polished, the whole electrodeposited copper foil is thinned to such an extent that the mechanical strength of wiring pattern or lead is poor. Further, when it is intended to carry out this chemical polishing, first, the shiny side is coated with a resist to thereby protect the same. Subsequently, the matte side is treated with a copper corrosive solution such as ferric chloride. After the treatment, the resist must be removed from the shiny side. Therefore, the preparing procedure is extremely time-consuming. Further, this chemical polishing poses a problem such that it is difficult to control a chemical polishing reaction so as to have the matte side uniformly treated. These problems of chemical polishing also occur in the electrolytic polishing involving leaching of copper.
Moreover, Japanese Patent Application Publication (Unexamined) No. Hei 3-296238 discloses a method of producing a TAB tape having a wiring pattern formed from a non-treated copper foil. The average surface roughness of the non-treated copper foil is described as falling within the range of 0.01 to 1 .mu.m.
However, the non-treated copper foil whose average surface roughness (Rz) falls within the range of 0.01 to 1 .mu.m, disclosed in this publication, is a rolled copper foil. The surface roughness of this non-treated rolled copper foil is too low to ensure sufficient peel strength (bonding strength). Accordingly, it is needed to preheat the copper foil or increase the diameter of the roller so as to form a covering film of cuprous oxide on the surface of the rolled copper foil. This poses a problem such that the process becomes laborious. Further, the use of this rolled copper foil renders it difficult to form a wiring pattern of extremely fine pitch such as one of not less than 30 .mu.m and less than 60 .mu.m pitch width.
Still further, Japanese Patent Application Publication (Unexamined) No. Hei 9-195096 discloses an invention directed to an electrodeposited copper foil for printed wiring board characterized in that the surface roughness (Rz) of the matte side of electrodeposited copper foil prior to nodulating treatment is not greater than 1.5 .mu.m while the surface roughness (Rz) after nodulating treatment on the matte side is in the range of 1.5 to 2.0 .mu.m. This electrodeposited copper foil is described as being producible by a method comprising buffing the matte side of an electrodeposited copper foil so as to cause the surface roughness (Rz) prior to nodulating treatment to become 1.5 .mu.m or less and subsequently effecting a nodulating treatment on the matte side so as to cause the surface roughness (Rz) to become 1.5 to 2 .mu.m.
However, the buffing as described in this publication may cause streaks on the buffed surface. These streaks result from polishing made deeper than predetermined. Some streaks have not posed any problem when use is made of the conventional thick electrodeposited copper foils. However, these streak portions indicate excess polishing of copper, so that, when use is made of thin copper foils, the mechanical strength of streak portions is extremely small. Thus, these are likely to become the cause of defective occurrence, for example, high possibility of open circuit at such portions in a wiring pattern or the like. Furthermore, in the execution of such buffing, stress is exerted on protrudent parts of the copper foil surface along the direction of buff rotation, so that protrudent parts of the copper foil surface are likely to deform along the direction of buff rotation. It is difficult to effect uniform nodulating treatment on the buffed copper foil having thus deformed protrudent parts. Nonuniform nodulating treatment would invite problems such that the lamination to insulating films, etching uniformity, bonding reliability, etc. are deteriorated. These problems are likely to occur especially when thin electrodeposited copper foils are mechanically polished.